"To the Assad regime, Russia, and Iran -- three Member States behind the conquest of and carnage in Aleppo -- you bear responsibility for these atrocities. By rejecting UN-ICRC evacuation efforts, you are signaling to those militia who are massacring innocents to keep doing what they are doing," Power said.

"Denying or obfuscating the facts, as you will do today -- saying up is down, black is white -- will not absolve you. When one day there is a full accounting of the horrors committed in this assault of Aleppo -- and that day will come, sooner or later -- you will not be able to say you did not know what was happening. You will not be able to say you were not involved. We all know what is happening. And we all know you are involved.

"Aleppo will join the ranks of those events in world history that define modern evil, that stain our conscience decades later. Halabja, Rwanda, Srebrenica, and, now, Aleppo.

Read More

"To the Assad regime, Russia, and Iran, your forces and proxies are carrying out these crimes. Your barrel bombs and mortars and airstrikes have allowed the militia in Aleppo to encircle tens of thousands of civilians in your ever-tightening noose. It is your noose. Three Member States of the UN contributing to a noose around civilians.

"Are you truly incapable of shame? Is there literally nothing that can shame you? Is there no act of barbarism against civilians, no execution of a child that gets under your skin?"

Russia fires back

JUST WATCHED

Brutal siege of Aleppo over?

MUST WATCH

Brutal siege of Aleppo over?00:10

Vitaly Churkin, Russia's ambassador to the UN, responded by accusing Power of acting like "Mother Teresa" despite America's "track record" in the Middle East.

"I wouldn't want to remind this Western trio [France, US, UK] which called for today's meeting and carried it out in a raised voice, about your role in the creation of ISIS as a result of US and UK intervention in Iraq," Churkin said.

"I don't want to remind these three countries about their role in unwinding the Syrian crisis, which led to such difficult consequences, and let terrorists spread in Syria and Iraq.

"The weirdest speech to me was the one by the US representative which built her statement as if she is Mother Teresa herself. Please, remember which country you represent. Please, remember the track record of your country."

Reports of mass executions

In recent days Syrian regime forces have undertaken a final bloody push to reclaim Aleppo from rebel forces, which have held the city for the last four years. But as the fighting has intensified, multiple reports have emerged accusing pro-government forces of carrying out mass executions.

The Syrian government has not commented on the reported killings, which include executions of women and children in formerly rebel-held areas, in state-run media.